I have a foxpro Inferno and an SP-60. I do notice the SP-60 gives a deeper/fuller/smoother/slightly louder sound than the inferno's speaker. 80% of the time for places I call, I just use the inferno by itself so i don't have to mess with an extra device and wire in the field. But i pull out the SP-60 if I want to boost the sound somewhere.
I haven't used it extensively, and I haven't tried the SP-70. I initially wanted the SP-70, but then decided on the SP-60. I don't remember why. With the SP-60, I didn't notice the performance gain over the internal speaker of the SP-60 that I was hoping for.